Send Message

What is machining accuracy? Interpretation of dimensional accuracy, shape accuracy and position accuracy

October 13, 2023

1. Dimensional accuracy

The degree to which the actual size of the processed part matches the center of the tolerance zone of the part size.


2. Shape accuracy

It refers to the degree to which the actual geometric shape of the machined part surface matches the ideal geometric shape.


3. Position accuracy

Refers to the actual position accuracy difference between the relevant surfaces of the processed parts.


4. Interrelationship

Usually when designing machine parts and specifying the machining accuracy of the parts, attention should be paid to controlling the shape error within the position tolerance, and the position error should be smaller than the dimensional tolerance. That is, for precision parts or important surfaces of parts, the shape accuracy requirements should be higher than the position accuracy requirements, and the position accuracy requirements should be higher than the dimensional accuracy requirements.


The deviation between the actual geometric parameters of the part and the ideal geometric parameters is called machining error. The size of the machining error reflects the level of machining accuracy. The larger the error, the lower the processing accuracy, and the smaller the error, the higher the processing accuracy.


Machining accuracy is mainly used to produce products. Machining accuracy and machining error are both terms for evaluating the geometric parameters of the machined surface. The machining accuracy is measured by the tolerance grade. The smaller the grade value, the higher the accuracy, the machining error is expressed by a numerical value. The larger the numerical value, the greater the error. High processing precision means small processing errors, and vice versa.


There are 20 tolerance levels from IT01, IT0, IT1, IT2, IT3 to IT18. Among them, IT01 represents the highest machining accuracy of the part, IT18 represents the lowest machining accuracy, and generally IT7 and IT8 have medium machining accuracy level.


The actual parameters obtained by any processing method will not be absolutely accurate. From the function of the part, as long as the processing error is within the tolerance range required by the part drawing, the processing accuracy is considered to be guaranteed.


5. Accuracy

Refers to the closeness between the obtained measurement results and the true value. High measurement accuracy means that the system error is small, when the average value of the measured data deviates less from the true value, but when the data is scattered, that is the magnitude of the accidental error is unclear.


6. Precision

It refers to the reproducibility and consistency between the results obtained by repeated measurements using the same spare sample. It is possible to have high precision, but accuracy is inaccurate. For example, using a length of 1mm to measure the three results are 1.051mm, 1.053, and 1.052. Although they have high precision, they are inaccurate.


Accuracy represents the correctness of the measurement results, and precision represents the repeatability and reproducibility of the measurement results. Precision is a prerequisite for accuracy.